AU3291800A - Catalysts for the gas-phase oxidation of ethylene and acetic acid to vinyl acetate and method for the production and use thereof - Google Patents

Description

WO 00/58008 PCT/EP00/02455 Catalysts for the gas-phase oxidation of ethylene and acetic acid to vinyl acetate, a process for producing them and their use 5 The present invention relates to highly selective catalysts for the synthesis of vinyl acetate from ethylene and acetic acid, to a process for producing them and to their use. 10 The preparation of vinyl acetate (VAM) in the gas phase from ethylene, acetic acid and oxygen or oxygen containing gases is known; the supported catalysts used for this synthesis comprise Pd as active metal and an alkali metal as promoter, preferably K in the form of 15 the acetate. Further additives used are Cd, Au or Ba. According to US-A-4 902 823, US-A-3 939 199, US-A-4 668 819 the catalytically active components are applied in finely divided form to the catalyst support 20 by impregnation, spraying on, vapor deposition, dipping or precipitation. The procedure described gives catalysts in which the active components have penetrated to the core of the support. 25 However, catalysts in which the active components have not penetrated to the core but are present only in a more or less thick outer part of the support particles, i.e. the shell of the support particles, are also known (EP-A-0 565 952, EP-A-0 634 214, EP-A-0 634 209, 30 EP-A-0 634 208). The catalysts known from the prior art comprise, as support, the known inert support materials such as silica, aluminum oxide, aluminosilicates, silicates, 35 titanium oxide, silicon carbide and carbon and are produced by subjecting the impregnated catalyst precursors to a reduction, e.g. in the gas phase at temperatures of 150-170 0 C, for example using ethylene or H 2 , or in the liquid phase at temperatures of < -2 100*C, e.g. using hydrazine. Higher reduction temperatures are deliberately avoided since appreciable sintering of the noble metal particles to form larger agglomerates having a lower catalytic activity occurs 5 at temperatures > 2000C. However, it is also known that the supports impregnated with the catalytically active components can be exposed to higher temperature ranges during the course of the 10 production of the finished catalysts. Thus, US-A-5 336 802 and US-A-5 194 417 describe the treatment of palladium- and gold-containing catalysts by single and multiple sequential oxidation and reduction steps. 15 EP-A-0 839 793 relates to a process for preparing vinyl acetate in the presence of a palladium-containing catalyst during whose production the impregnated catalyst precursor which has been treated with a 20 reducing agent is subjected to an additional sintering step at a temperature in the range from 500 to 10000C. WO 98/18553 discloses calcining a palladium- and gold containing catalyst after the impregnation step at a 25 temperature of from 100 to 6000C in a nonreducing atmosphere and only then carrying out the reduction step. The calcination can also be carried out in an oxygen atmosphere. 30 In the processes disclosed in EP-A-0 839 793 and WO 98/18553, the preparation of vinyl acetate is also carried out using supported catalysts based on inert support materials such as silica, aluminum oxide, aluminosilicates, silicates, titanium oxide, silicon 35 carbide and carbon. In view of the fact that catalysts are used in processes carried out on an industrial scale, for example the preparation of vinyl acetate, there is -3 great interest in improving the catalysts, particularly in respect of their activity and selectivity. It is therefore an object of the present invention to 5 provide catalysts for the synthesis of vinyl acetate, which catalysts have a particularly high activity, selectivity and operating life. A further object of the present invention is to provide 10 catalysts which have a particularly high stability. In particular, the catalysts should be insensitive to local temperature fluctuations and hot spots and also to fluctuations in the oxygen concentration during the reaction in the synthesis of vinyl acetate and to 15 mechanical stress. These objects are achieved by a catalyst comprising palladium, at least one alkali metal compound and, if desired, one or more promoters on a porous support, 20 which catalyst is obtainable by loading the porous support in which at least one reducible metal oxide is present with at least one palladium compound, subsequently carrying out a reduction at a temperature of > 2000C and additionally applying at least one 25 alkali metal compound and, if desired, one or more promoters before or after the reduction. The invention accordingly provides a catalyst comprising palladium, at least one alkali metal 30 compound and, if desired, one or more promoters on a porous support and obtainable by loading the porous support in which at least one reducible metal oxide is present with at least one palladium compound, subsequently carrying out a reduction at a temperature 35 of > 2000C and additionally applying at least one alkali metal compound and, if desired, one or more promoters before or after the reduction.

-4 The invention further provides a process for producing catalysts, which comprises loading the porous support in which at least one reducible metal oxide is present with at least one palladium compound, subsequently 5 carrying out a reduction at a temperature of > 200*C and additionally applying at least one alkali metal compound and, if desired, one or more promoters before or after the reduction. 10 The invention further provides a process for preparing vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases in the presence of a catalyst comprising palladium, at least one alkali metal compound and, if desired, one or more 15 promoters on a porous support and obtainable by loading the porous support in which at least one reducible metal oxide is present with at least one palladium compound, subsequently carrying out a reduction at a temperature of > 2000C and additionally applying at 20 least one alkali metal compound and, if desired, one or more promoters before or after the reduction. In the following, the reduction of the porous support in which at least one reducible metal oxide is present 25 and which is laden with palladium compounds at a temperature of > 2000C is referred to as the high temperature reduction and abbreviated as HTR. An essential aspect of the catalysts of the invention 30 is the use of a support material in which at least one reducible metal oxide is present, for example oxides of the elements of groups IIIb, IVb, Vb, VIb of the Periodic Table of the Elements, ZnO and other reducible oxides known from the literature and also mixtures of 35 these reducible metal oxides. Furthermore, the mixed oxides derived from the abovementioned oxides are also suitable. Preference is given to TiO 2 and ZrO 2 having intermediate to high specific surface areas as support material.

-6 from 0.2 to 1.2 ml/g. The pore volume is determined by means of mercury porosimetry. The support materials employed according to the 5 invention can be produced by any method known to those skilled in the art. They are generally commercially available. Loading a reducible support with at least one palladium 10 compound and subsequently carrying out a high temperature reduction at temperatures of > 200 0 C and additionally applying at least one alkali metal compound and, if desired, one or more promoters before or after the reduction gives catalysts which have an 15 excellent activity and selectivity and also excellent adhesion of noble metal to the support. The novel measure of the HTR of porous reducible supports laden with palladium compounds is carried out 20 at temperatures of > 200 0 C, preferably > 300 0 C. If desired, one or more promoters are subsequently applied to the support which has been loaded in this way. Preference is given to using at least one Au, Ba 25 or Cd compound or a mixture of these compounds. However, the addition of promoters is preferably carried out before the HTR treatment, either in admixture with the palladium compound or compounds or 30 separately therefrom. The order plays no role; the porous reducible support can be loaded first with the palladium compound and then with the promoters or vice versa. 35 The catalysts of the invention further comprise at least one alkali metal compound, preferably at least one potassium compound. The addition of alkali metal to the porous reducible support can be carried out prior to the HTR treatment, either separately or in admixture -5 However, it is also possible to use inert support materials such as silica, aluminum oxide, aluminum silicates, silicates, silicon carbide or carbon as long as these inert materials are first coated with the 5 abovementioned reducible metal oxides. Such support materials are known from EP-A-723 810. The reducible metal oxide content is from 0.1 to 25% by weight, based on the total support material. 10 It is likewise possible to use mixtures of the reducible metal oxides with these inert materials as supports for the catalysts of the invention. Here, the proportion of the inert materials can be up to 75% by weight, based on the total support material. The 15 proportion of the inert materials is preferably up to 50% by weight, based on the total support material. Support materials in which at least one reducible metal oxide is present are hereinafter referred to as 20 reducible support materials. Surprisingly, nonreducible support materials known from the prior art, for example silica or aluminum oxide or mixtures thereof, are unsuitable for producing the 25 catalysts of the invention. Oxides which cannot be reduced to the metal under the production conditions employed according to the invention are likewise unsuitable as support materials. 30 The reducible support can be used either in powder form or as shaped bodies. The support material is preferably used as shaped bodies and is in the form of pellets, spheres, tablets, rings, extrudates, ribbed extrudates, star extrudates, stars, hollow extrudates or other 35 industrial shaped bodies. The diameter or the length and thickness of the support materials is generally from 3 to 9 mm. The surface area of the support measured by the BET method is generally 10-500 m 2 /g, preferably 15-250 m 2 /g. The pore volume is generally -7 with the other promoters and/or with the palladium compound or compounds, or after the HTR treatment, either separately or in admixture with the other promoters. 5 Finally, the catalysts of the invention can be finished in accordance with the prior art and used in industrial processes, for example in the synthesis of vinyl acetate. 10 According to the invention, the reducible support material is first a palladium compound. The type of compound is not critical here, as long as high dispersion of the metal can be achieved. Soluble 15 palladium compounds are suitable for this purpose, in particular water-soluble salts. Preference is given to, for example, palladium compounds selected from the group consisting of palladium(II) acetate, palladium(II) chloride, sodium tetrachloropalladate(II) [Na 2 PdCl 4 ] and 20 palladium(II) nitrate. Besides palladium(II) acetate, further carboxylates of palladium can be used, preferably the salts of aliphatic monocarboxylic acids having from 3 to 5 carbon atoms, for example the propionate or butyrate. 25 However, in the case of the chlorides, it has to be ensured that the chloride ions are removed to leave a tolerable residual amount before use of the catalyst. For this purpose, the catalyst support is washed with 30 water after loading with palladium compounds and with the promoters and after the HTR. This is achieved by washing the support, e.g. with water, after Pd and, if used, Au have been fixed to the support by reduction to the metals. 35 The palladium compounds mentioned can be prepared by any method known to those skilled in the art. However, most of them are also commercially available.

-8 Promoters used are compounds of the elements Au, Ba and Cd, in particular soluble salts of these elements. Suitable gold compounds include, for example, 5 tetrachloroauric(III) acid [HAuCl 4 ], gold(III) acetate [Au(OAc) 3 ], potassium aurate [KAuO 2 ]. It is advisable to prepare the gold acetate or the potassium aurate freshly by precipitation of the oxide/hydroxide from solutions of tetrachloroauric acid, washing and 10 isolating the precipitate and taking it up in acetic acid or KOH. The gold compounds are preferably applied to the support before the high-temperature reduction. Suitable cadmium compounds include, for example, 15 cadmium acetate Cd(OAc) 2 and other carboxylates of cadmium, for example the propionate or butyrate. The cadmium compounds can be applied to the support either before or after the high-temperature reduction. 20 Suitable barium compounds include, for example, barium acetate Ba(OAc) 2 , other barium carboxylates such as the propionate or butyrate and also barium hydroxide Ba(OH) 2 . The barium compounds can be applied to the support either before or after the high-temperature 25 reduction. The catalyst of the invention further comprises at least one alkali metal compound, preferably at least one potassium or rubidium compound and particularly 30 preferably at least one potassium compound. Suitable potassium compounds include, for example, potassium acetate KOAc, potassium carbonate K 2

CO

3 , potassium hydrogen carbonate KHCO 3 and potassium hydroxide KOH, and also all potassium compounds which are converted 35 into the acetate under the reaction conditions. The potassium compounds can be applied to the support either before or after the high-temperature reduction.

-9 Preference is given to using the acetates for loading the porous reducible support since these catalysts cause very little chloride contamination. 5 The metal compounds are usually used in concentrations of from about 0.1 to 100 g per liter, preferably from 1 to 50 g per liter, based on the solvent. Suitable solvents are all compounds in which the 10 compounds or salts chosen are soluble and which can be easily removed again by drying after the impregnation. Examples of suitable solvents for the acetates are, in particular, unsubstituted carboxylic acids, especially acetic acid, while the chlorides are soluble primarily 15 in water or dilute hydrochloric acid. If the salts are not sufficiently soluble in acetic acid or in water, the additional use of a further solvent in addition to water or acetic acid, which are 20 also used as a mixture, can be advantageous. Possible additional solvents are ones which are inert and miscible with acetic acid or water. Examples of additions to acetic acid are ketones such as acetone 25 and acetylacetone, also ethers such as tetrahydrofuran or dioxane, acetonitrile, dimethylformamide and also hydrocarbons such as benzene. The application of the active component Pd and the 30 further promoters and also the alkali metal compound can be carried out by the methods known from the prior art. The finished catalysts have the following metal 35 contents, expressed in gram of metal per 1 1 of finished catalyst: Palladium: generally 1 - 20 g/l preferably 3 - 15 g/l in particular 5 - 10 g/l - 10 Alkali metal content: generally 5 - 30 g/l preferably 10 - 25 g/l in particular 10 - 16 g/l 5 If the finished catalyst further comprises one or more promoters, the promoter content is in each case generally up to 20 g/l, preferably 2-15 g/l and in particular 3-10 g/l. 10 In the synthesis of vinyl acetate, preference is given to using the catalyst systems based on Pd/Cd/K, Pd/Ba/K or Pd/Au/K. Particular preference is given to systems comprising Pd-Au-K. It is an essential aspect of the present invention that at least Pd and possibly Au are 15 subjected to the HTR, while the other promoters such as the potassium compounds and further additives can be added either before or after the HTR. It is found that the catalysts of the invention are 20 superior to the catalysts known from the prior art in respect of activity and particularly in respect of the selectivity in industrial processes. An example of an important industrial process of this type is the synthesis of vinyl acetate. 25 The use of the catalysts of the invention in the synthesis of vinyl acetate has, in particular, the following advantages: 30 Surprisingly, a selectivity increase of more than 5% is achieved, which is measurable by a drastic reduction in the undesirable total oxidation to C0 2 . At the same time, an activity increase of more than 20% 35 compared to catalysts known from the prior art is achieved. The novel measure of the HTR induces a very strong interaction between the noble metal particles and the - 11 reducible support and this interaction effects an advantageous change in the properties of the noble metal particles for catalysis and is responsible for excellent mechanical anchoring to the support and thus 5 a high agglomeration resistance. The high-temperature-reduced catalysts of the invention have a particularly uniform Pd/Au active metal distribution and a high degree of dispersion of the 10 noble metals. The high degree of dispersion is largely maintained even in long-term operation owing to reduced agglomeration of the noble metal particles, as a result 15 of which the deactivation of the catalysts of the invention is slowed and long operating lives are achieved. In addition, the catalysts of the invention have 20 excellent mechanical stabilities since the chemically reactive reducible supports can easily be shaped to produce mechanically hard shaped bodies and the HTR at high temperatures further increases the hardness of the shaped bodies. 25 The catalysts of the invention are insensitive to hot spots and also to fluctuations in the oxygen concentration during the reaction of the vinyl acetate synthesis, and the intensity of the hot spots is 30 greatly reduced owing to the extraordinarily high selectivity (drastic reduction in the strongly exothermic total oxidation to CO or C0 2 ), as a result of which regulation and process control are significantly simplified. 35 Since an increase in the space-time yield (STY) accompanies a selectivity increase, particularly high throughputs can be realized in large plants.

- 12 Achievement of the abovementioned advantages could not have been foreseen from the prior art, since the prior art puts forward the opinion that reduction temperatures of > 200 0 C are damaging and, owing to 5 agglomeration of the noble metal particles, reduce the activity. Thus, hot spots of > 190*C are avoided if possible both in the conventional reduction and later in operation. 10 Furthermore, the prior art teaches the use of inert supports such as silicon dioxide or silicon dioxide/aluminum oxide mixtures, preferably silicon dioxide. 15 Loading of the support with the desired amounts of the respective compounds can be carried out in one step or a plurality of sequential steps, with drying steps being able, if desired, to be inserted between the individual steps. 20 It is possible, for example, to apply one compound, e.g. a salt, of each of the elements to be applied to the support particles, e.g. Pd/K/Au, Pd/K/Cd or Pd/K/Ba. However, a plurality of salts of one element 25 can also be applied or mixed compounds of the various metals can be used. In general, exactly one salt of each of the three elements is used. The salts can be applied to the support by known 30 loading methods, for example steeping, impregnation, spraying on, vapor deposition, dipping or precipitation. The support can be impregnated right through with the salts or it is possible to use all methods known to those skilled in the art which are 35 employed for producing surface-impregnated catalysts. These are disclosed, for example, in the documents DE-A-1 668 088, US-A 3 775 342, US-A 3 822 308, US-A 4 048 096, US-A 5 185 308, US-A 5 567 839, - 13 US-A 5 314 858, EP-A-0 634 208, EP-A-0 634 209 or EP-A-0 634 214. In the case of the Pd/Au/K catalysts, it has been found 5 to be advantageous to apply the two noble metals in the form of a shell on the support, i.e. the noble metals are distributed only in a zone close to the surface while the regions further inside the support body are virtually free of noble metals. The thickness of this 10 catalytically active shell is usually 0.1-2 mm. In many cases, surface-impregnated catalysts make possible a more selective process than is the case for catalysts in which the support particles are 15 impregnated to the core ("fully impregnated"). If the reaction conditions when using surface impregnated catalysts are left unchanged from the reaction conditions when using fully impregnated 20 catalysts, more vinyl acetate can be produced per unit reactor volume and time, which equates to a capacity increase without additional capital costs. The work-up of the crude vinyl acetate obtained is also made easier, since the vinyl acetate content of the product 25 gas from the reactor is higher, which also leads to an energy saving in the work-up. Suitable work-up processes are described, for example, in US-A 5 066 365, DE-34 22 575, DE-A-34 08 239, DE-A-29 45 913, DE-A-26 10 624, US-A 3 840 590. 30 If, on the other hand, the plant capacity is kept constant, the reaction temperature can be lowered and the reaction can therefore be carried out more selectively at the same total output, thus saving 35 starting materials. This also reduces the amount of carbon dioxide which is formed as by-product and therefore has to be discharged and decreases the loss of entrained ethylene associated with this discharge.

- 14 Furthermore, this operating procedure leads to a lengthening of the operating life of the catalyst. According to the invention, the catalyst has to be 5 subjected to a high-temperature reduction after loading with at least one palladium compound. For this purpose, a vaporizable or gaseous reducing agent can be passed at temperatures of above 200*C, 10 preferably in the range from 2000C to 7000C and particularly preferably in the range from 300*C to 600*C, over the loaded catalyst. Suitable reducing agents for the HTR are all materials 15 which are able to reduce the palladium compounds and, if used, gold compounds to the metals at the high reduction temperatures employed according to the invention. 20 Preference is given to gaseous or vaporizable reducing agents for example H 2 , CO, ethylene, NH 3 , formaldehyde, methanol and hydrocarbons in general and also mixtures of these reducing agents. Hydrogen is particularly preferred. 25 The gaseous reducing agents can also be diluted with inert gas, for example carbon dioxide, nitrogen or argon. Preference is given to using a reducing agent diluted with inert gas. Mixtures of hydrogen with 30 nitrogen or argon, preferably those having a hydrogen content of from 1% by volume to 15% by volume, are preferred. The reduction times are generally in the range from 1 35 minute to 24 hours, particularly preferably from 30 minutes to 10 hours. The amount of reducing agent is selected so that at least the number of equivalents of reducing agent - 15 required for complete reduction of the noble metals are passed over the catalyst during the treatment time. Preference is given to passing an excess of reducing agent over the catalyst in order to ensure complete 5 reduction. The volume flow of reducing gas is calculated from the pressure employed in the reduction, the dilution and the reaction time. Preference is given to carrying out the reduction at atmospheric pressure, i.e. at an absolute pressure of about 1 bar. For the 10 production of industrial amounts of the catalyst of the invention, preference is given to using a rotary tube furnace or a fluidized-bed reactor in order to ensure uniform reduction of the catalyst. 15 In the reduction according to the invention, it is first and foremost the noble metal compounds, i.e. Pd and for example Au, which are reduced to the corresponding metals, with the support material being partially reduced, e.g. to form Ti 3 + centers in the TiO 2 20 lattice. In contrast thereto, the other metal compounds present, i.e. the alkali metal compounds and the other promoters apart from gold, are generally not reduced. For this reason, the alkali metal compounds and the nonreducible promoters can be applied to the support 25 either before or after the high-temperature reduction. In a preferred embodiment of the invention, TiO 2 or ZrO 2 supports or mixtures thereof or mixtures with further inert support constituents, for example SiO 2 and/or 30 A1 2 0 3 , preferably as shaped bodies, more preferably in the form of pellets, spheres, rings, extrudates or tablets, are impregnated with palladium acetate and gold acetate, dried and reacted with gaseous reducing agents, preferably H 2 , ethylene and/or NH 3 , in the HTR 35 in the temperature range of 200-700*C, preferably 300 600*C, for a period ranging from 1 minute to 10 hours. If desired, this is followed by impregnation with potassium acetate and final drying at a temperature of - 16 not more than 1500C, preferably 80-150*C and in particular 100-150*C. However, it is also possible to use support materials 5 in which the inert support constituents have firstly been coated with TiO 2 or ZrO 2 or with a mixture of these oxides. This is followed by the impregnation step and the HTR. The proportion of TiO 2 or Zr0 2 is from 0.1 to 25% by weight, based on the support material. 10 Further pretreatment or after-treatment steps which are known to those skilled in the art can be inserted. These include, inter alia, washing, drying, calcination, oxidation and/or reduction. 15 The catalysts of the invention are preferably used for preparing vinyl acetate. This is generally carried out by passing acetic acid, ethylene and oxygen or oxygen containing gases at temperatures of from 100 to 220*C, 20 preferably from 120 to 200*C, and pressures of from 1 to 25 bar, preferably from 1 to 20 bar, over the finished catalyst, with unreacted components being able to be circulated. The oxygen concentration is advantageously kept below 10% by volume (based on the 25 gas mixture excluding acetic acid). However, dilution with inert gases such as nitrogen or carbon dioxide is sometimes also advantageous. Carbon dioxide is particularly suitable for dilution, since it is formed in small amounts during the reaction. The vinyl acetate 30 formed is isolated by means of suitable methods as are described, for example, in US-A 5 066 365, DE-A-34 22 575, DE-A-34 08 239, DE-A-29 45 913, DE-A-26 10 624, US-A 3 840 590. 35 The following examples serve to explain and illustrate the invention without restricting it thereto.

- 17 Example 1 2.11 g of palladium acetate (224.49 g/mol) and 1.32 g of gold acetate (374.10 g/mol) were dissolved in 30 ml 5 of glacial acetic acid. The preparation of gold acetate is described, for example, in US 4,933,204. 100 ml of TiO 2 supports (P25 pellets, DEGUSSA) were added to this solution. Subsequently, the major part of the glacial acetic acid was firstly distilled off at 700C on a 10 rotary evaporator, residues of solvent were then removed at 600C with the aid of an oil pump and finally in a vacuum drying oven for a period of 14 hours, likewise at 600C. 15 The reduction was carried out using a gas mixture of 10% by volume of H 2 in N 2 . Here, the gas was passed at a temperature of about 500 C through the pellets for one hour. The reduction was carried out under atmospheric pressure at a flow of 40 1/h of a mixture of 10% by 20 volume of H 2 in nitrogen. To load them with potassium ions, the pellets were treated with a solution of 4 g of potassium acetate in 30 ml of water. This mixture was allowed to act on the pellets at room temperature for 15 minutes in a mixer. The solvent was subsequently 25 removed on a rotary evaporator. The pellets were dried for 14 hours at 1100C in a drying oven. The catalyst comprised: 7 g/l Au; 16 g/l K; <1 g/l Cl; 10 g/l Pd. 30 The production of the catalyst was repeated twice. The testing of these catalysts in the synthesis of vinyl acetate is reported as Examples 1A to 1C in Table 1 below. 35 Example 2 2.11 g of palladium acetate (224.49 g/mol) and 1.32 g of gold acetate (374.10 g/mol) were dissolved in 45 ml - 18 of glacial acetic acid. 100 ml of TiO 2 supports (XT25376 pellets, Norton) were added to this solution. Subsequently, the major part of the glacial acetic acid was firstly distilled off at 70*C on a rotary 5 evaporator, residues of solvent were then removed at 600C with the aid of an oil pump and finally in a vacuum drying oven for a period of 14 hours, likewise at 600C. 10 The reduction was carried out using a gas mixture consisting of 10% by volume of H 2 in N 2 . Here, the gas was passed at a temperature of about 500*C through the pellets for 1 hour. The reduction was carried out under atmospheric pressure at a flow of 40 1/h of a mixture 15 of 10% by volume of H 2 in nitrogen. To load them with potassium ions, the pellets were treated with a solution of 4 g of potassium acetate in 45 ml of water. The pellets which had been impregnated in this way were mixed for 10 minutes at room temperature. The solvent 20 was subsequently removed on a rotary evaporator. The pellets were dried for 14 hours at 1100C in a drying oven. The catalyst comprised: 7 g/l Au; 16 g/l K; <1.1 g/l 25 Cl; 10 g/l Pd. Examples 3A-C 2.11 g of palladium acetate (224.49 g/mol), 1.32 g of 30 gold acetate (374.10 g/mol) and 4.0 g of potassium acetate were dissolved in 30 ml of glacial acetic acid. 100 ml of TiO 2 supports (P25 pellets, DEGUSSA) were added to this solution. Subsequently, the major part of the glacial acetic acid was firstly distilled off at 35 600C on a rotary evaporator and residues of solvent were then removed at 600C in a vacuum drying oven over a period of 4 hours.

- 19 The batch was divided into three parts which were subjected to different reduction conditions. _Batch Time Temperature Reducing agent A 4 h 4000C 10% by volume of H 2 in N 2 B 4 h 4500C 10% by volume of H 2 in N 2 C 4 h 500*C 10% by volume of H 2 in N 2 5 The reduction was carried out using a gas mixture consisting of 10% by volume of H 2 in N 2 . Here, the gas was passed at a temperature of about 4000C (batch A), 450 0 C (batch B) or 5000C (batch C) through the pellets for 4 hours in each case. The reduction was carried out 10 under atmospheric pressure at a flow of 40 1/h. The catalyst comprised: 7 g/l Au; 16 g/l; <1.5 g/l Cl; 10 g/l Pd. 15 Example 4 1.06 g of palladium acetate (224.49 g/mol), 0.66 g of gold acetate (374.10 g/mol) and 2.0 g of potassium acetate were dissolved in 15 ml of glacial acetic acid. 20 50 ml of TiO 2 supports (P25 pellets, DEGUSSA) were added to this solution. Subsequently, the major part of the glacial acetic acid was firstly distilled off at 600C on a rotary evaporator and residues of solvent were then removed at 60 C for 14 hours in a vacuum 25 drying oven. The reduction was carried out at 5000C for 1 hour using a gas mixture consisting of 10% by volume of H 2 in N 2 , using a method similar to Example 1. 30 The catalyst contained: 7 g/l Au; 16 g/l K; <1 g/l Cl; 10 g/l Pd. Example 5 - 20 1.06 g of palladium acetate (224.49 g/mol), 0.66 g of gold acetate (374.10 g/mol) and 4.0 g of potassium acetate were dissolved in 30 ml of glacial acetic acid at 60'C. 100 ml of TiO 2 supports (P25 pellets, DEGUSSA) 5 were added to this solution. Subsequently, the major part of the glacial acetic acid was firstly distilled off at 600C on a rotary evaporator and residues of solvent were then removed at 600C for 4 hours using an oil pump. 10 The reduction was carried out at 500*C for 1 hour using a gas mixture consisting of 10% by volume of H 2 in N 2 , using a method similar to Example 1. 15 The catalyst contained: 3.5 g/l Au; 16 g/l K; <0.5 g/l Cl; 5 g/l Pd. Comparative example 1 20 1.82 g of Na 2 PdCl 4 (294.19 g/mol) and 0.64 g of NaAuCl 4 (361.76 g/mol) were dissolved in 32 ml of demineralized water. This solution was applied completely to 100 ml of SiO 2 supports (KA160 pellets, Sid-Chemie) with gentle agitation. To form a noble metal shell, the 25 pretreated support was placed in a solution of 0.85 g of sodium hydroxide, NaOH, in 32 ml of demineralized water. The reaction mixture was left to stand overnight and the pellets were then washed free of chloride using demineralized water. 30 The catalyst was then reduced with an ethylene/nitrogen mixture (5% of ethylene in nitrogen) at 1500C for 5 hours. 35 To load the pellets with potassium ions, they were treated with a solution of 4 g of potassium acetate in 30 ml of water and the finished catalyst was dried for 2 hours in a rapid drier.

- 21 The catalyst comprised: 3.5 g/l Au; 16 g/l K; <0.5 g/l Cl; 6.6 g/l Pd. Comparative example 2 5 1.06 g of palladium acetate (224.49 g/mol), 0.7 g of gold acetate (374.10 g/mol) and 4.0 g of potassium acetate were dissolved in 30 ml of glacial acetic acid at 60*C. 100 ml of TiO 2 supports (P25 pellets, DEGUSSA) 10 were added to this solution. Subsequently, the major part of the acetic acid was firstly distilled off at 60 0 C on a rotary evaporator and residues of solvent were then removed at 60 0 C over a period of 4 hours using an oil pump. 15 Reduction was carried out using a gas mixture consisting of 10% by volume of H 2 and 90% by volume of

N

2 , in each case based on the volume, at 170 0 C for 1 hour (atmospheric pressure, gas flow: 40 1/h). 20 The catalyst comprised: 3.5 g/l Au; 16 g/l K; <0.5 g/l Cl; 5 g/l Pd. Comparative example 3 25 0.53 g of palladium acetate (224.49 g/mol), 0.33 g of gold acetate (374.10 g/mol) and 2.0 g of potassium acetate were dissolved in 30 ml of glacial acetic acid. 50 ml of TiO 2 supports (XT25376 pellets, Norton) were 30 added to this solution. Subsequently, the major part of the glacial acetic acid was firstly distilled off at 70 0 C on a rotary evaporator, residues of solvent were then removed at 60*C with the aid of an oil pump and finally in a vacuum drying oven for a period of 14 35 hours, likewise at 60*C. The reduction was carried out thermally without reducing gas (autoreduction) . Here, nitrogen as flushing gas was passed through the pellets for 1 hour - 22 at a temperature of about 5000C (atmospheric pressure, 40 1/h) . The catalyst comprised: 3.5 g/l Au; 16 g/l K; <0.5 g/l 5 Cl; 5 g/l Pd. Comparative example 4 1.06 g of palladium acetate (224.49 g/mol), 0.7 g of 10 gold acetate (374.10 g/mol) and 4.0 g of potassium acetate were dissolved in 80 ml of glacial acetic acid at 600C. 100 ml of SiO 2 supports (Aerosil 200 pellets, DEGUSSA) were added to this solution. Subsequently, the major part of the glacial acetic acid was firstly 15 distilled off at 60*C on a rotary evaporator and residues of solvent were then removed at 60*C over a period of 4 hours using an oil pump. The reduction was carried out using a gas mixture 20 consisting of 10% by volume of H 2 in N 2 for 1 hour at 5000C, using a method similar to Example 1. The catalyst comprised: 3.5 g/l Au; 16 g/l K; <0.5 g/l Cl; 5 g/l Pd. 25 Comparative example 5 1.06 g of palladium acetate (224.49 g/mol), 0.7 g of gold acetate (374.10 g/mol) and 4.0 g of potassium 30 acetate were dissolved in 80 ml of glacial acetic acid at 60*C. 100 ml of SiO 2 supports (KA160 pellets, Sud Chemie) were added to this solution. Subsequently, the major part of the glacial acetic acid was firstly distilled off at 600C on a rotary evaporator and 35 residues of solvent were then removed at 60*C over a period of 4 hours using an oil pump.

- 23 The reduction was carried out using a gas mixture consisting of 10% by volume of H 2 in N 2 for 1 hour at 500*C, using a method similar to Example 1. 5 The catalyst comprised: 3.5 g/l Au; 16 g/l K; <0.5 g/1 Cl; 5 g/l Pd. Reactor tests for the gas-phase oxidation of ethylene and acetic acid to vinyl acetate: 10 The catalysts produced in the examples and comparative examples are tested in a fixed-bed tube reactor having a tube diameter of 2 cm. The reactor is heated externally by means of oil jacket heating. The reactor 15 is typically charged with 15 ml of the shaped catalyst bodies. The reactor volume upstream and downstream of the catalyst bed is filled with glass spheres to reduce the dead volume. The gases are meted in via mass flow regulators for gases; the acetic acid is meted in using 20 a mass flow regulator/vaporizer unit. The gases and the acetic acid are mixed in a packed gas mixing tube. The test apparatus is operated continuously. The reaction is monitored continually by means of a gas 25 chromatograph. When a steady-state reaction has been achieved, i.e. the reactor temperature is constant and the concentrations of vinyl acetate and C02 in the product 30 gas stream are constant, the recording of data is commenced. The experiments were carried out using a reaction temperature in the range 150-170*C and a reaction 35 pressure of 8-9 bar. The composition of the starting material stream was typically 60-80% by volume of ethylene, 10-20% by volume of N 2 , 10-20% by volume of acetic acid and 2-10% by volume of 02. A complete analysis of the output from the reactor was carried out - 24 directly at the reactor outlet by means of on-line GC (2 column arrangement) and on-line IR. The GC data were used to calculate the vinyl acetate 5 selectivities S (= mol of VAM/(mol of VAM + 0.5* mol of Cox, X = 1 or 2) and STY (space-time yield = g of VAM/l of cat. *h). Table 1 shows the results of the individual experiments in which the catalysts prepared in Examples 1 to 5 were tested. 10 Table 1: Testing of the catalysts in the synthesis of vinyl acetate Example T p 02 conc. S STY No. (0C) (bar) (%) % g/l*h 1A 170 9 5.2 96 1000 160 9 5.2 98 1050 155 9 5.2 98 1000 1B 170 9 5.2 97 700 160 9 5.2 98 1150 1C 170 9 5.2 98 1300 2 170 9 5.2 96 1200 3A 160 9 5.2 89 1400 150 9 5.2 98 1400 3B 160 9 5.2 95 1260 3C 160 9 5.2 96 1210 4 150 9 5.2 96 1100 5 160 9 5.2 95 940 Comp.1 170 9 5.1 88 850 Comp.2 160 9 5.2 80 870 Comp.3 170 9 5.2 77 <50 Comp.4 167 9 5.2 89 190 Comp.5 170 9 5.2 83 340 15 It can be seen from the table that the HTR treatment of catalysts in which a reducible support material is present significantly improves the selectivity and - 25 yield in the vinyl acetate synthesis compared to the catalysts known from the prior art.

Claims (23)

1. A catalyst comprising palladium, at least one alkali metal compound and, if desired, one or more 5 promoters on a porous support, obtainable by loading the porous support in which at least one reducible metal oxide is present with at least one palladium compound, subsequently carrying out a reduction at a temperature of > 200*C and 10 additionally applying at least one alkali metal compound and, if desired, one or more promoters before or after the reduction.

2. A catalyst as claimed in claim 2 which comprises 15 at least one potassium compound.

3. A catalyst as claimed in claim 1 or 2 which additionally comprises Au, Ba and/or Cd and/or their compounds as promoters. 20

4. A catalyst as claimed in one or more of claims 1 to 3, wherein the reducible metal oxide is an oxide selected from among the oxides of elements of groups IIIb, IVb, Vb, VIb of the Periodic Table 25 of the Elements or ZnO or is a mixture of these oxides or a mixed oxide of these elements in which zinc may also be present.

5. A catalyst as claimed in any one of claims 1 to 4, 30 wherein the porous support further comprises an inert support material.

6. A catalyst as claimed in any one of claims 1-5, wherein the porous support comprises silica, 35 aluminum oxide, aluminosilicates, silicates, silicon carbide or carbon or a mixture thereof as inert support material. - 27

7. A catalyst as claimed in one or more of claims 1 to 6, wherein the reducible support is TiO 2 .

8. A catalyst as claimed in one or more of claims 1 5 to 7, wherein the reduction is carried out in the temperature range from 2000C to 7000C, in particular from 300 0 C to 6000C.

9. A catalyst as claimed in one or more of claims 1 10 to 8, wherein the reduction is carried out for a time in the range from 1 minute to 24 hours.

10. A catalyst as claimed in one or more of claims 1 to 9, wherein the reduction is carried out using 15 gaseous or vaporizable reducing agents.

11. A catalyst as claimed in one or more of claims 1 to 10, wherein the reducing agent for the reduction is selected from the group consisting of 20 H 2 , CO, ethylene, NH 3 , formaldehyde, methanol, hydrocarbons and their mixtures and mixtures of these reducing agents with inert gases.

12. A process for producing catalysts, which comprises 25 loading the porous support in which at least one reducible metal oxide is present with at least one palladium compound, subsequently carrying out a reduction at a temperature of > 2000C and additionally applying at least one alkali metal 30 compound and, if desired, one or more promoters before or after the reduction.

13. The process as claimed in claim 12, wherein the catalyst comprises at least one potassium 35 compound.

14. The process as claimed in claim 12 or 13 wherein the catalyst additionally comprises Au, Ba and/or Cd and/or their compounds as promoters. - 28

15. The process as claimed in one or more of claims 12 to 14, wherein the reducible metal oxide is an oxide selected from among the oxides of elements of groups IIIb, IVb, Vb, VIb of the Periodic Table 5 of the Elements or ZnO or is a mixture of these oxides or a mixed oxide of these elements in which zinc may also be present.

16. The process as claimed in any one of claims 12 to 10 15, wherein the porous support further comprises an inert support material.

17. The process as claimed in any one of claims 12 to 16, wherein the porous support comprises silica, 15 aluminum oxide, aluminosilicates, silicates, silicon carbide or carbon or a mixture thereof as inert support material.

18. The process as claimed in one or more of claims 12 20 to 17, wherein the reducible support is TiO 2

19. The process as claimed in one or more of claims 12 to 18, wherein the reduction is carried out in the temperature range from 2000C to 700 0 C, in 25 particular from 3000C to 6000C.

20. The process as claimed in one or more of claims 12 to 19, wherein the reduction is carried out for a time in the range from 1 minute to 24 hours. 30

21. The process as claimed in one or more of claims 12 to 20, wherein the reduction is carried out using gaseous or vaporizable reducing agents. 35

22. The process as claimed in one or more of claims 12 to 21, wherein the reducing agent for the reduction is selected from the group consisting of H 2 , CO, ethylene, NH 3 , formaldehyde, methanol, - 29 hydrocarbons and their mixtures and mixtures of these reducing agents with inert gases.

23. The use of a catalyst obtainable as claimed in any 5 of claims 12-22 for the preparation of vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases.